The present invention resides in a method of incinerating waste material which needs to be treated thermally, such as garbage, on a grate of an incineration apparatus wherein, for performing the method in accordance with the principle of parallel flow heating, primary air is admitted to the grate from below to a combustion chamber.
Thermal treatment of waste material is important in the framework in the concept of integrated waste material economics. However, waste material is still burnt with a relatively large amount of excess air. More than 3 Nm.sup.3 air are required per kg of combustible material with a heat content of about 8 MJ/Kg. Actually, 6 Nm.sup.3 have been used until not long ago. By this time, the specific air use number has been reduced to about 5 Nm.sup.3.
Cost efficient waste material incineration plants have not been developed since, up to this date, no processes have become available which would prevent the formation of NO.sub.x at the primary combustion site to such a degree that no exhaust gas cleaning equipment is required in the exhaust gas flue which does not need any means for reducing the NO.sub.x emissions. Although there is a limit value of 200 mg NO.sub.x /Nm.sup.3 for the combustion of waste materials the public expects the NO.sub.x emission values to be less than half that amount. Consequently, the design target to be achieved for the primary side NO.sub.x emission reduction is 100 mg/Nm.sup.3.
U.S. Pat. No. 3,808,986 discloses an apparatus for the incineration of waste material. The purpose and the design for this apparatus are to increase the combustion temperatures in order to reduce the amount of the components which can normally not be burned. This leads to exhaust gas temperatures of much over 1000.degree. C. and consequently to the formation of relatively large amounts of NO.sub.x in the exhaust gas. This however is not acceptable with the ever stricter exhaust gas quality requirements. Other apparatus known in the art which operate in a center- and counter current flow fashion generally have high NO.sub.x emission values in the range of 200 to over 400 mg/Nm.sup.3.
Further incineration processes and apparatus using a parallel flow principle for the combustion of waste materials are known from DE 42 19 231 C1 and from Thome-Kozmiensky: Thermische Abfallbehandlung, Berlin, EF Verlag fur Energie-und Umwelttechnik, 1994, pages 160 to 163. In this process wherein secondary air is supplied to the combustion chamber from the top, a temperature profile is generated in the combustion zone above the grate which provides for uniformly increasing temperatures of from 700.degree. C. at the beginning of the grate to 1300.degree. C. at the end of the combustion zone ahead of the combustion gas exhaust downstream of the combustion chamber area. Such an arrangement however also leads to undesirably high NO.sub.x emission values which was not recognized and generates a problem that has not been addressed by those designs.
It is therefore the object of the present invention to provide a method with which the NO.sub.x content in the exhaust gas of such incinerators is reduced purely by measures to the combustion chambers. The invention is based on the understanding that this can be achieved with a temperature reduction to below 900.degree. C. at the end of the combustion chamber where the exhaust gas leaves the combustion chamber. Even such an object is novel.